While both radial flow and uniform flow are desirable attributes of a diffuser output, neither is ordinarily obtained in prior art devices. Owing to strong logarithmic flow exiting the impeller, the flow tends to separate from the suction side of vanes in the diffuser and is likewise biased towards the pressure surface of the vanes. Even at the design mass flow point, prior art diffusers are characterized by turbulence and strong whirl at the exit.
Prior art remedies have included the provision of a second full size set of vanes, either radially outwardly from the first set or radially inwardly as in the case where said vanes are placed in the return channels of a multi-stage compressor after a right angle bend. These prior art solutions are generally ineffective because they must contend with a highly disturbed flow, or costly because full size vanes further increase the size of the diffuser.